1. Field of Invention
This invention relates to the preparation of aromatic amines and more particularly pertains to an improved process for the preparation of polymethylene polyphenylpolyamine mixtures by the catalyzed condensation reaction of aniline and formaldehyde.
2. Description of the Prior Art
Known processes for preparing methylene-bridged polyphenylpolyamines by the catalyzed condensation reaction of aniline and formaldehyde at elevated temperatures are amply described in the literature and many patents, for example U.S. Pat. Nos. 2,683,730; 2,950,263; 3,012,008; 3,344,162; 3,362,979; 3,277,173; and 3,496,229. Generally speaking, known procedures for preparing polymethylene polyphenylpolyamine mixtures include mixing and reacting aniline and formaldehyde at elevated temperatures in the presence of an acidcontaining material as a catalyst for the reaction. The resulting polymethylene polyphenylpolyamine reaction product mixture contains a diamine portion, i.e., diaminodiphenylmethane, and higher functionality, higher molecular weight methylene-bridged polyphenylpolyamines. The diamine portion of the reaction product mixture can be substantially controlled by varying several of the reaction condition variables, such as the molar ratio of aniline to formaldehyde employed. Moreover, as known, the positional isomer content of the diamine portion of the polyphenylpolyamine reaction product, namely the 2,2'-, 2,4'-, and 4,4'-diaminodiphenylmethane isomers can be varied by varying reaction conditions and employing certain types of acidic material-containing catalysts.
For example, known processes employing strong mineral acids, such as hydrochloric acid, usually result in a polyamine reaction product mixture having high 4,4'- positional isomer content in the diamine portion. Such polyamine reaction product mixtures are particularly useful as precursors for the preparation of corresponding polyisocyanate mixtures employed in the production of certain types of elastomers, coatings, structural materials, etc.
As another example, known procedures employing solid acidic siliceous catalysts, such as silica-alumina cracking catalyst, result in the production of polyamine reaction products having diamine portions which contain higher levels of 2,2'-, and 2,4'-isomers and less 4,4'-isomer, based upon the diamine portion, than the aforementioned mineral catalyzed procedures. These polyamine reaction product mixtures are also useful as precursers for preparation of corresponding polyisocyanate mixtures which are particularly useful in the production of certain types of coatings, elastomers, and the like.
The aforementioned conventional procedures for preparing methylene-bridged polyphenylpolyamine mixtures by the acid-catalyzed condensation reaction of aniline and formaldehyde, however, suffer from several disadvantages. Strong mineral acids such as hydrochloric acid, are hgihly corrosive and procedures employing same require the use of special corrosion resistant equipment. Moreover, the employment of mineral acids generally requires neutralization of the crude condensation reaction product mixture with a basic material such as caustic to produce the desired polymethylene polyphenylpolyamine mixtures. Neutralization brings on the additional difficulties of by-product salt removal and disposal.
Conventional procedures employing solid-acidic siliceous catalysts such as silica-alumina do not suffer from corrosiveness problems and neutralization of the crude condensation reaction product is not necessary. Yet solid acidic siliceous catalyst procedures suffer from other disadvantages such as slow condensation reaction rates. In order to increase the reaction rates these catalysts, particularly silica-alumina, are employed in solid very finely powdered form in amounts of aout 3.0 weight percent, basis aniline present. However, such use has brought on the attendant difficulty of separating these solid, powdered catalysts from the crude condensation reaction product mixtures. In addition, it has been very difficult to prepare polymethylene polyphenylpolyamine mixtures containing a diamine portion having a high 4,4'- positional isomer content by such procedures. The resulting polymethylene polyphenylpolyamine mixtures must be subjected to extensive distillation procedures so as to obtain such mixtures having high 4,4'- isomer containing diamine portions.
I have now discovered a process for preparing methylene-bridged polyphenylpolyamine mixtures by the catalyzed condensation reaction of aniline and formaldehyde at elevated temperatures which does not suffer from any of the aforementioned disadvantages. Moreover, the process of my invention results in the production of a methylene-bridged polyphenylpolyamine mixture having a diamine portion containing high levels of the 4,4'positional isomer which is highly useful in the preparation of epoxy curing agents and urethane cross-linkers and as precursors for preparing corresponding polymethylene polyphenylpolyisocyanate mixtures which can be used to produce certain types of coatings, elastomers, rigid and flexible foams, and the like. The invention is based upon the discovery of a novel co-catalyst system for the aniline-formaldehyde condensation reaction.